Pulse reverse current high rate electrodeposition and charging while mitigating the adverse effects of dendrite formation

ABSTRACT

The problem of high rate electrodeposition of metals such as copper during electrowinning operations or high rate charging of lithium or zinc electrodes for rechargeable battery applications while avoiding the adverse effects of dendrite formation such as causing short-circuiting and/or poor deposit morphology is solved by pulse reverse current electrodeposition or charging whereby the forward cathodic (electrodeposition or charging) pulse current is “tuned” to minimize dendrite formation for example by creating a smaller pulsating boundary layer and thereby minimizing mass transport effects leading to surface asperities and the subsequent reverse anodic (electropolishing) pulse current is “tuned” to eliminate the micro- and macro-asperities leading to dendrites.

RELATED APPLICATIONS

This application claims benefit of and priority to U.S. ProvisionalApplication Ser. No. 62/727,105 filed Sep. 5, 2018, under 35 U.S.C. §§119, 120, 363, 365, and 37 C.F.R. § 1.55 and § 1.78, which isincorporated herein by this reference.

GOVERNMENT RIGHTS

This invention was made with Government support under contractFA8650-19-P-2024 awarded by USAF/AFMC, AFRL Wright Research Site. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

This subject invention relates to high rate electrodeposition processessuch as electrowinning in general and to high rate charging of batteriesin particular.

BACKGROUND OF THE INVENTION

Everyone desires faster charging lithium batteries. The market isworldwide and includes commercial (portable power and electric vehicles)and military applications.

The market for lithium-ion batteries in 2010 was reported to beapproximately $11 billion, with projections of $43 billion by 2020, and$93.1 billion by 2025. The growth is largely attributed to expanded useof batteries in personal electronic devices as well as demands fromemerging grid storage and transportation markets. In particular, theglobal aerospace and defense market is projected to grow at a compoundannual growth rate (CAGR) of 4.8% and forecasted to reach $200 millionby 2022.

Additionally, the vehicle/automotive sector has experienced the greatestdemand growth due to favorable incentives and technological innovation,leading to fast electrified vehicle adoption as well as expansion of DoDcapabilities for surveillance and reconnaissance. Considerationsprompting battery innovation for automotive applications includebalancing safety, cost, and performance for the next-generation of“advanced” lithium-ion technology, with these advanced Li-ion batteriesprojected to overtake current Li-ion battery's market share by 2027.However, in order for electrified vehicles to be economically viable ona global scale, a target of <$150/kWh must be achieved to compete withinternal combustion engines (S100/kWh).

Of secondary interest is high rate metal electrowinning from miningoperations.

There is high interest in lithium batteries in general and high rateelectrodeposition (charging) in particular. Recently, papers suggestingpulse charging have been reported as an approach to eliminate dendriteformation during high rate charging. See Yang et al., Effects of PulsePlating on Lithium Electrodepository Morphology, and Cycling Efficiency,Journal of Power Sources 272 (2014) 900-908 incorporated herein by thisreference. These approaches have been unsuccessful and are not likely tosucceed as dendrite formation is a stochastic process and can likelynever be completely and assuredly eliminated.

Although studied for at least 50 years, a complete and detailedmechanistic understanding of dendrite growth remains elusive. Generally,during electrodeposition (or charging of batteries), stochasticprocesses lead to localized surface roughness or imperfections on theplated surface. These local imperfections represent high current densityareas for subsequent material deposition resulting in dendrite growth.The growth of dendrites is accelerated as the electrodeposition rateapproaches the limiting current density as evident by the resultantfractal patterns. Limiting current density denotes the maximum faradaiccurrent density and occurs when the rate of charge transfer exceeds rateof reactant mass transport to the electrode surface. Once the limitingcurrent density is reached, the rate of faradaic reactions approach amaximum since the reactant concentration approaches zero at the surface.Thus, methods of increasing the limiting current density wouldnecessarily involve controlling the concentration gradients near theelectrode surface.

In conventional copper electroplating, chemical additions are includedto address issues of copper deposit roughness. A chemical additiveapproach analogous to copper plating has been considered for avoidanceof lithium dendrite formation however, the continual replenishment ofchemical additions is challenging for sealed batteries throughout theoperational lifetime. Specifically, in metal plating, the chemicaladditives are continually replenished in the plating bath untileventually the bath is “dumped” and a new bath is established. This isnot practical for lithium batteries and since dendrite formation is astochastic process, a certain amount of dendrite formation will occureven with a well-designed additive package. Furthermore, for someapplications such as metal electrowinning where high rateelectrodeposition is desired for economic reasons, the addition ofadditives is not practical.

Mechanistic understanding of dendrite formation related to lithiumbattery distinguish between two scales and mechanisms leading todendrite growth:

-   1) Whiskers—needle-like filaments that grow from their roots at    charging current densities below the limiting current,-   2) Dendrites—“classical” fractal structures which grow at their tips    at charging current densities approaching the limiting current.

While significant research efforts towards mitigating lithium dendriticstructures, including electrolyte additives, stabilization ofsolid-electrolyte interphase (SEI), suppression of lithium whiskers anddendrites and curbing capacity loss resulting from the accretion ofelectrochemically inactive or “dead” lithium at practical currentdensities and areal capacities remains elusive. Recent findings indicatesuccessful mitigation of dendritic features by embedding lithiumthroughout high surface area, porous substrates. These substrates allowfor high rate charging and areal capacities—attributed to the highsurface area in contact with the electrolyte. In addition, thesesubstrates allow for the confinement of lithium electrodeposits withinthe bulk pore structure, due to the lower effective local currentexperienced throughout the entire electrode. Lastly, porous substratesoffer an excellent three-dimensional (3D) framework to facilitate chargetransfer—especially when considering the intimate contact between theactive electrodeposited lithium with the substrate. Although employingporous substrates appear to be a promising strategy towards mitigatingdendritic features through confinement within bulk pore structures, thethreat of lithium whiskers and dendrites persists at the outer surfaceof the porous electrode, where lithium dendritic features are notconfined and free to grow.

BRIEF SUMMARY OF THE INVENTION

Disclosed herein is a high rate electrodeposition processes for metalelectrowinning and charging processes for secondary batteries includinglithium and zinc systems that avoid the detrimental impact of dendriteformation.

The inventors and common assignee have pioneered the use of pulsereverse electrolytic principles for electrodeposition in electrolytesdevoid of additives. Specifically, commonly assigned patents disclosepulse reverse electroplating processes for copper and other metals thatdo not use chemical additives as described in U.S. Pat. Nos. 6,203,684;6,210,555; 6,303,014; 6,309,528; 6,319,384; 6,524,461; 6,551,485;6,652,727; 6,750,144; 6,827,833; 6,863,793; 6,878,259; 8,603,315; all ofwhich are incorporated by reference. In addition, the inventors andcommon assignee have pioneered the use of pulse reverse electrolyticprinciples for electropolishing, as disclosed in U.S. Pat. Nos.6,402,931; 6,558,231; 7,022,216; 9,006,147; 9,987,699; all of which areincorporated by reference.

The instant invention addresses the problem of dendrite formation duringhigh rate electrodeposition for electrowinning applications of metalssuch as copper and charging of secondary batteries such as lithium andzinc by tuning the cathodic electrodeposition or charging pulse tominimize the formation of asperities leading to dendrites followed bytuning the anodic electropolishing pulse to remove the micro- ormacro-asperities formed during the previous cathodic pulse.

In an embodiment of the invention, the subject electrodeposition andelectropolishing pulses are sequenced one after the other.

In another embodiment of the invention, off-times are inserted after theelectrodeposition pulses.

In still another embodiment of the invention, off-times are insertedafter the electropolishing pulses.

In still another embodiment of the invention, off-times are insertedafter both the electrodeposition and electropolishing pulses.

In still another embodiment of the invention, the electrodeposition andelectropolishing pulses are tuned to minimize and eliminatemicro-asperities.

In still another embodiment of the invention, the electrodeposition andelectropolishing pulses are tuned to minimize and eliminatemacro-asperities.

In still another embodiment of the invention, the electrodeposition andelectropolishing pulses are sequenced to minimize and eliminate firstmicro-asperities and then macro-asperities.

In still another embodiment of the invention, the electrodeposition andelectropolishing pulses are sequenced to minimize and eliminate firstmacro-asperities and then micro-asperities.

In still another embodiment of the invention, the electrodeposition andelectropolishing pulses are sequenced and then looped to minimize andeliminate first macro-asperities and then micro-asperities.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a generalized pulse reverse waveform;

FIG. 2 is a representation of pulsating boundary layer versus astationary DC boundary layer for (A) cathodic electrodeposition andcharging pulse and (B) anodic electropolishing pulse;

FIG. 3 is a representation of generalized waveform parameter and theirinfluence on current distribution under microprofile and macroprofileconditions;

FIG. 4 is a representation of a surface with a micro-asperity and thegeneralized waveform parameters for its minimization and removal;

FIG. 5 is a representation of a surface with a macro-asperity and thegeneralized waveform parameters for its minimization and removal;

FIG. 6 is a representation of surface with micro-asperities andmacro-asperities and the generalized waveform parameters applied insequence or loops for its minimization and removal;

FIG. 7 is a schematic view of a battery being charged in accordance withthe waveform parameters of FIG. 4, 5, or 6;

FIG. 8 is a schematic view of an electrowinning process in accordancewith the waveform parameters of FIG. 4, 5, or 6;

FIG. 9 is an exploded view of a coin cell;

FIG. 10 is a graph showing voltage transients from conventional DCcycling;

FIGS. 11A-1I B are a graph showing voltage transients from pulse reversecharging corresponding to one waveform; and

FIGS. 12A-12C show voltage transients from pulse reverse chargingcorresponding to another waveform.

DETAILED DESCRIPTION OF THE INVENTION

Aside from the preferred embodiment or embodiments disclosed below, thisinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Thus, it is to be understood that theinvention is not limited in its application to the details ofconstruction and the arrangements of components set forth in thefollowing description or illustrated in the drawings. If only oneembodiment is described herein, the claims hereof are not to be limitedto that embodiment. Moreover, the claims hereof are not to be readrestrictively unless there is clear and convincing evidence manifestinga certain exclusion, restriction, or disclaimer.

In FIG. 1, we present a generic pulse reverse current waveform for a netcathodic or charging (plating) process. The generic descriptionillustrates a cathodic (forward) pulse followed by an off-time, followedby an anodic (reverse) pulse and followed by a second off-time. Thecathodic peak current density (i_(cathodic)), cathodic on-time(t_(cathodic)), cathodic off-time (t_(off,cathodic)), anodic peakcurrent density (i_(anodic)), anodic on-time (t_(anodic)), and anodicoff-time (t_(off,anodic)) are individual variables for process control.There are numerous embodiments of pulse and pulse reverse currentwaveforms, a common one being waveforms consisting of only cathodicpulses. Additionally, other common embodiments that include the anodicpulse may eliminate one or both off-times. The sum of the cathodicon-time, anodic on-time, and off-time(s) is the period (T) of the pulseand the inverse of the period is the frequency (f). Specifically,T=(t _(cathodic))+(t _(off,cathodic))+(t _(anodic))+(t_(off,anodic))  (1)f=(1/T)  (2)The cathodic duty cycle (γ_(cathodic)) is the ratio of the cathodicon-time to the pulse period, and the ratio of the anodic on-time to thepulse period is the anodic duty cycle (γ_(anodic)). The frequency andduty cycles are additional variables for process control. The averagecurrent density (i_(average)) or electrodeposition rate is given by:i _(average)=(i _(cathodic))(γ_(cathodic))−(i_(anodic))(γ_(anodic))  (3)

It should be noted that even though pulse current and pulse reversecurrent (PC/PRC) waveforms contain off-times and anodic periods, the netplating (i.e. charging) rate is typically the same or higher than indirect current plating. As discussed below, this is attributed to thefact that the “instantaneous” peak currents attained during the pulseon-time can be much higher than that attained during DC plating.

Mass transport in pulse current plating is a combination of steady stateand non-steady state diffusion processes. The mass transfer limitedcurrent density (i_(t)) is related to the reactant concentrationgradient (C_(b)-C_(s)) and to the diffusion layer thickness (δ) by:i _(t) =−nFD(∂C/∂x)_(x=0) =−nFD[(C _(b)-C _(s))/δ]  (4)where n, F, D are the number of equivalents exchanged, Faraday'sconstant, and diffusivity of the reacting species, respectively. In DCelectrolysis, δ is a time-invariant quantity for a given electrodegeometry and hydrodynamic condition. In pulsed electrolysis, however, δvaries from 0 at the beginning of the pulse to its steady state valuewhen the Nernst diffusion layer, δ_(N), is fully established. Thecorresponding mass transport limiting current density is equal to aninfinite value at t=0 and decreases to a steady state value of the DClimiting current density. The advantage of pulse electrolysis is thatthe current is interrupted before δ reaches steady state, allowing thereacting ions to diffuse back to the surface and partially or completelyreplenish the surface concentration to its original value before thenext current interruption. Therefore, the concentration of reactingspecies in the vicinity of the electrode changes with the pulsefrequency. During pulse electrolysis, a “duplex diffusion layer”consisting of a pulsating layer, δ_(p), and a stationary layer, δ_(s)for a deposition process (FIG. 2a ). FIG. 2b shows the equivalentdiffusion layers for an anodic process, e.g. metal removal. By assuminga linear concentration gradient across the pulsating diffusion layer andconducting a mass balance, the pulsating diffusion layer thickness(δ_(p)) is:δ_(p)˜(2Dt _(on))^(1/2)  (5)where t_(on) is the pulse on-time. When the pulse on time is equal tothe transition time, the concentration of reacting species at theinterface drops to zero at the end of the pulse. An expression for thetransition time, τ, is:τ˜((nF)² C _(b) ² D)/2i _(c) ²  (6)

For on-times less than the transition time, τ, the concentration of thereacting metal species at the interface remains above zero. For on-timesequal to or greater than τ, the concentration of the reacting metalspecies at the interface is zero, and the process is mass transportlimited.

For a DC process, a hydrodynamic Nernst diffusion layer is established.The thickness of the pulsating diffusion layer is related to the pulsecurrent on-time and we refer to it as the “electrodynamic” diffusionlayer. The key points used in the development of a pulsed process forelectrodeposition are:

-   (1) the electrodynamic diffusion layer thickness is proportional to    pulse on time, and-   (2) transition time is inversely proportional to the pulse current.

As mass transport is implicated in the dendrite formation and growthmechanism, the use of pulse reverse electrolytic processes is animportant benefit.

Current distribution is an important parameter in electrodeposition orbattery charging processes. Primary current is governed solely by thegeometric effects of the electrochemical cell. Secondary currentdistribution is governed by kinetic effects and activationoverpotentials are considered. Tertiary current distribution is governedby mass transport effects and both activation and concentrationoverpotentials are considered. The addition of secondary or tertiarycurrent distribution effects tend to make the current distribution moreuniform, as compared to primary current distribution alone. For thepurposes of this discussion, it is helpful to examine the balance ofprimary and secondary current distribution in terms of the Wagnernumber. The Wagner number (W_(a)) is the ratio of the activationpolarization (kinetic effects) to the ohmic polarization (geometriceffects):W _(a)=(dη/di)·(κ/l)  (7)where “dη/di” is the Tafel slope for the reaction of interest, “κ” isthe solution conductivity, and “l” is the characteristic length overwhich the electrochemical reaction is assumed to take place. ForW_(a)<<1, primary current distribution prevails and the currentdistribution is less uniform.ForW_(a)>>1, secondary current distribution prevails and the currentdistribution is more uniform. According to the Wagner numberrelationship, the tools with which to understand and perhaps manipulatethe balance of primary, secondary and tertiary current distribution are:

-   -   Tafel slope    -   Solution conductivity    -   Characteristic length

The characteristic length, l, may change over time as the plated surfaceis roughened or smoothed. The solution conductivity is a function ofelectrolyte chemistry and temperature. Manipulation of the Tafel slopemay be done using pulsed electric fields.

A further consideration with regard to current distribution and thebalance of primary, secondary and tertiary effects is the relationshipbetween the characteristic length, l, and the diffusion layer. In amacroprofile, the roughness of the surface is large compared with thethickness of the hydrodynamic diffusion layer, δ_(H), and when a pulsedelectric field is applied, the diffusion layer is compressed to form apulsating or electrodynamic diffusion layer, δ_(p). The pulsatingdiffusion layer, δ_(p), tends to follow the surface contour, and becomesmore compressed and thinner as the pulse on-time becomes shorter. In amicroprofile, the roughness of the surface is small compared with thethickness of the hydrodynamic diffusion layer, δ_(H). In this case, fora long pulse on-time, the pulsating diffusion layer is compressed, butstill is much larger than the characteristic length, and themicroprofile is maintained. For very short pulse on-times, the pulsatingdiffusion layer is compressed to the point at which it follows thesurface contour, and the system effectively mimics a macroprofile.

A final consideration is that if the applied waveform is designed suchthat the pulse on time is much longer than the transition time, tertiarycurrent distribution will play an important role. With the addition oftertiary control, the concepts of macroprofile and microprofile andtheir influence on current distribution become important. Under DCconditions and mass transport control, a macroprofile results in themost uniform current distribution and a nearly conformal deposit. Theapplication of pulse currents generates a smaller macroprofile. Based onexperimental observations, for a macroprofile boundary layer condition,relatively long pulse on-times can yield a slightly non-uniform currentdistribution compared to DC conditions, and relatively short pulseon-times can yield a significantly more non-uniform current distributionthan DC conditions. Assuming the same average current, for shorter pulseon-times the relative influence on current distribution shifts fromtertiary current distribution control to secondary as well as primarycurrent distribution control. Consequently, as concentrationpolarization effects are removed, the current distribution becomes lessuniform.

Under DC conditions and mass transport control, a microprofile resultsin the most non-uniform current distribution and a non-conformaldeposit. The application of pulse currents with a small enough on-timecan convert a microprofile to a macroprofile, establishing a smallδ_(p). For a microprofile diffusion layer condition, assuming tertiarycurrent distribution control is maintained, short pulse on-timessufficient to convert the microprofile to a macroprofile results in asignificantly more uniform current distribution. Conversely, long pulseon-times sufficient to maintain the microprofile results in a slightlymore uniform current distribution compared to DC, assuming tertiarycontrol is maintained by selecting on-times and peak currents thatensure to t_(on)>>τ.

In the case of high rate electrodpeposition or battery charging, weanticipate the occurrence of both micro-asperity whisker andmacro-asperity dendrites. The whiskers may represent a microprofilerelationship to the boundary layer and the dendrites may represent amacroprofile relationship to the boundary layer. Consequently, byapplying the appropriately designed current pulse in terms of peakcurrent and on-time, the current distribution can be effectively focusedor de-focused during electrodeposition or charging.

While these concepts serve as “guiding” principles for our selection anddevelopment of specific pulse waveform parameters for specificapplication, they do not provide a priori guidance for waveformparameter selection. Rather, an amount of limited experimentation isrequired. We have taken these concepts and have developed a paradigm forwaveform parameter selection in terms of current distribution formacroprofile and microprofile condition. There are four pulse waveformtypes, independent of anodic or cathodic orientation that we generallyuse to tune waveforms for specific applications, and these aresummarized in FIG. 3. While there are additional permutations that maybe considered, but these are the ones that have proven most useful inour development activities. For electrochemical processes that are closeto 100% faradaic efficiency for both metal deposition and metaldissolution, combining these cathodic and anodic pulses can create netwaveforms that will achieve the overall desired process result.Combining multiple waveforms into sequences may also be required, aswill be shown in the examples to follow. This is easily done with modemprogrammable rectifiers—a far simpler change than having to modifyelectrolytes or electrode geometries to achieve the same effect.

Grain size in electrodeposited coatings, and hence mechanical propertiessuch as ductility, is also a function of the pulse current platingparameters. During the electrodeposition or charging process, as ionsenter the electrified interface between the solution and the cathode, acharge transfer reaction results in the formation of adatoms on thesurface of the cathode. Electrocrystallisation is the mechanism by whichadatoms are incorporated into a crystal lattice during plating.Electrocrystallization can occur by either growth on previouslydeposited crystals or nucleation of new crystals. Ifelectrocrystallization occurs by growth on previously depositedcrystals, the resulting deposit will consist of large grains. Incontrast, if electrocrystallization occurs by nucleation of newcrystals, the resulting deposit will consist of small grains or evenamorphous deposits. The nucleation rate (ν) is given by:ν=k ₁ exp(−k ₂/|η|)  (8)where k₁ is proportionality constant, k₂ is related to the amount ofenergy needed for the two-dimensional nucleation, and η is theoverpotential. The nucleation rate increases exponentially withincreasing overpotential.

Pulse current plating generally utilizes peak current densities that aremuch higher than the current densities used in DC plating. Therefore,the instantaneous current or voltage pulses, and hence theoverpotentials during pulse current plating, may be higher than duringDC plating. Consequently, pulse current plating can promote nucleationand a finer grained structure compared to DC plating. In fact, we havetaken the “grain size” concept to the extreme by forming 4 to 5 nmcatalyst particles for gas diffusion electrode applications as describedin U.S. Pat. Nos. 6,080,504; 5,084,144; all of which are incorporated byreference. The impact of grain size during high rate electrodepositionand battery charging also have an effect on whisker and dendriteformation.

In FIG. 4, we illustrate the whisker and type of waveform parameters ofinterest for the microprofile charging case. Note, the average chargingrate, compensating for reverse/off times, will be tuned to be greaterthan the DC charging rate of ˜20 mA/cm². Based on the discussion above,the forward cathodic pulse is tuned to deposit lithium with minimalsurface imperfections, i.e. whiskers, under microprofile conditions.However, as surface imperfections, in particular whiskers are assumed tobe a stochastic process, they will occur. Consequently, the reverseanodic pulse is tuned to selectively remove the whiskers, in essencepolishing the surface.

In FIG. 5, we illustrate the growing dendrite and type of waveformparameters of interest for the macroprofile charging case. Note, theaverage charging rate, compensating for reverse/off times, will be tunedto be greater than the DC charging rate of ˜20 mA/cm². Based on thediscussion above, the forward cathodic pulse is tuned to deposit lithiumwith minimal surface imperfections, i.e. whiskers, under microprofileconditions. However, as surface imperfections, in particular whiskersare assumed to be a stochastic process, they will occur. Dendrites willgrow from the whiskers. Consequently, the reverse anodic pulse is tunedto selectively remove the dendrites, in essence polishing the surface.An off-time or relaxation period may follow either of both of thecathodic and anodic pulses shown in FIG. 4 and FIG. 5.

The cathodic charge (Q_(cathodic)) density is:Q _(cathodic)=(i _(cathodic))(t _(cathodic))  (9)

The anodic charge (Q_(anodic)) density is:Q _(anodic)=(i _(anodic))(t _(anodic))  (10)

The cathodic to anodic charge ratio is:Q _(cathodic) /Q _(anodic)  (11)

The ordinate in FIG. 4 and FIG. 5 is cathodic (−) and anodic (+) and mayrepresent either current or voltage although battery charging andelectrowinning operations are generally conducted under current control.

The electrolyte (battery electrolyte or electrowinning electrolyte) ispreferably substantially devoid of levelers, brighteners and otheradditives commonly known in the plating industry to control depositgrain size (and hence mechanical properties) and/or create a more leveldeposit. Note: in the plating industry, these additives are replenishedbased the amount of metal deposited (charge passed) from the subjectplating bath. See U.S. Pat. No. 6,319,384 incorporated herein by thisreference.

Featured as shown in FIG. 4 is a method for depositing a layer of metalresulting in a smooth metal deposit devoid of dendrites from anelectrolyte using a pulse reverse current waveform to eliminatemicroasperities. In some examples, Li, and/or Zn is used for high-ratebattery charging and Cu for high-rate electrowinning.

The preferred metal electrolyte is substantially devoid of levelers,brighteners and other additives. Microasperities are less than 100microns.

A preferred cathodic to anodic Charge ratio is >1, preferably >10,preferably >50, preferably >100. A preferred average Current Densityis >5 mA/cm², preferably >10 mA/cm², preferably >20 mA/cm². A preferredfrequency is 0.10 to 1000 Hz. A preferred cathodic duty cycle is <50%,preferably <30%, preferably <10%. A preferred anodic duty cycle is >50%,preferably >70%, preferably >90%.

According to the method of FIG. 5, it is preferred that anymacroasperities are >100 microns; the cathodic to anodic charge ratiois >1, preferably >10, preferably >50, preferably >100; the averagecurrent density is >5 mA/cm², preferably >10 mA/cm², preferably >20mA/cm²; the frequency is 0.10 to 1000 Hz; the cathodic duty cycleis >50%, preferably >70%, preferably >90%; and the anodic duty cycle is<50%, preferably <30%, preferably <10%.

FIG. 7 shows a battery 10 charged by a charger 12 configured to applythe waveforms of FIG. 4, 5, or 6. FIG. 8 shows an electrowinning processwhere charger 14 applies the waveform of FIG. 4, 5, or 6.

Finally, we anticipate that during the time-scales desired for longcycle life lithium batteries, we anticipate the lithium surface maycontain both whiskers (nascent dendrites) and growing dendrites.Consequently, we may find it desirable to sequence and loop the pulsereverse electrodeposition and charging waveforms between microprofileand macroprofile parameters as illustrated in FIG. 6.

Pulse Reverse charging was explored as an approach for lithium dendritemitigation during high rate charging. Coin cells containing symmetriclithium anode/lithium cathode (Li/Li) were constructed to demonstratethe Pulse Reverse Charging concept. FIG. 9 depicts the coin cellcomponents where the electrolyte-soaked separator 20 is flanked on bothsides by lithium metal electrodes 22 a, 22 b. While a lithiumcathode/lithium anode configuration is not a battery couple, by usinglithium as both electrodes, both charge and discharge cycles impacteddendrite formation on the anode and cathode, respectively. In otherwords, “charge” is defined as electrochemical reduction of lithium ionsonto the anode. Conversely, discharge is the electrochemical oxidationof lithium from the anode. The other components (spacer, washer) serveto impart mechanical pressure and ensure intimate contact while maintaina hermetically sealing environment contained within the cap/gasket/can.

Various waveform parameters, “A”, “B”, and “C”, listed in Table 1, wereemployed in the following examples to demonstrate conventional directcurrent (DC) charging and Pulse Reverse Charing. Current densities inTable 1 (˜7 mA/cm²) reflect significantly higher rates than typicallyrecommended charging rates (200 μA/cm² to 1 mA/cm²) for commercial coincells. The resulting voltage transients were examined for indications ofdendrites. Monitoring the voltage transient for instabilities, such asvoltage spikes or fluctuations, is often leveraged as an indirectindicator of lithium

TABLE 1 Waveform parameters for coin cell charge/discharge cycling.Pulse Total Wave- Current density duration duration form(mA/cm{circumflex over ( )}2) (s) (s) Notes A  7 N/A 3600 DC Charge −7N/A 3600 DC Discharge B 50 (forward) 0.02 3600 Pulse Reverse −3.5(reverse) 0.08 Charge 3.5 (forward) 0.08 3600 Pulse Reverse −50(reverse) 0.02 Discharge C 10.6 (forward) 0.08 3600 Pulse Reverse −7(reverse) 0.02 Charge 7 (foward) 0.02 3600 Pulse Reverse −10.6 (reverse)0.08 Dischargedendrite and the onset of short-circuit formation.

Example I

FIG. 10 depicts the typical DC charge (designated as the positivevoltage segments) and corresponding DC discharge (negative voltagesegments) transient for a Li/Li symmetrical coin cell and is designatedas “Waveform A” in Table 1. The voltage transient is a reflection of thedriving force which is manifested as an overpotential, or voltage biasbetween the two electrodes, necessary to overcome the resistances inorder to initiate and sustain the electrodeposition (orelectrodissolution) process at a given electrochemical current. Thus,any significant deviations in the resistance experienced between theanode/cathode, for example resulting from a dendrite-facilitatedshort-circuit, would be manifested as a voltage fluctuation. FIG. 10depicts cycles 1, 3, and 5 of the (+/−) 7 mA/cm² charge (positivevoltages) and discharge (negative voltages) of a Li/Li symmetric coincell. It is evident that cycles 1 and 3 exhibited a smooth voltagetransient during both charge and discharge of the cell. However, incycle 5, voltage fluctuations during the discharge segment wasattributed to lithium dendrites and the onset of short-circuit. Effortsto inhibit or mitigate dendrites focused on controlling the lithium ionconcentration at the surface of the lithium electrode through variousPulse Reverse waveforms.

Example II

FIG. 11A shows a representative voltage response from applying a pulsereverse “Waveform B” (Table 1) to a Li/Li symmetric coin cell. Thecurrent amplitude and duration of the forward and reverse pulses wereconstrained to yield a net charge passed (in coulombs, for example),which was equivalent to the analogous DC case (FIG. 10 corresponding to“Waveform A” in Table 1) for both the coin cell charge and dischargecycles. Thus, a comparable amount of lithium was deposited and removedvia both the conventional DC as well as the Pulse Reverse cyclingstrategies within the same period of time. By adjusting the x-axisscale, voltage fluctuations experienced during “Waveform B” can beobserved in FIG. 11B. Notably in FIG. 11B the voltage fluctuations weredelayed until the charge segment of the 6th cycle. In other words,evidence of lithium dendrites and the onset of short circuit was delayeduntil the 6th cycle and support the claim that controlling the lithiumion concentration at the lithium metal electrode via Pulse/Pulse Reversewaveforms can inhibit/mitigate lithium dendrites.

Application of Pulse Reverse charging delays the lithium dendriteformation from the 5th to the 6th cycle or a 20% increase in cycle life.Noted that in these tests the Li/Li symmetric coin cells were cycled atrelatively extreme current densities of (+/−) 7 mA/cm². With morerealistic cycling conditions, the dendrite mitigation benefits impartedby Pulse Reverse charging would be amplified since lower currentdensities would generally promote fewer asperities throughout theoperational lifetime of the cell. Specifically, Pulse Reverse chargingover the span of hundreds/thousands of cycles would tune the lithiummetal electrode surface topography by iteratively and selectivelyelectropolishing the asperities and mitigating other unavoidablesubsurface precursors leading to lithium dendrites.

Example III

It should also be emphasized that not all pulse reverse waveforms areappropriate for dendrite mitigation FIG. 12 is an example of anotherpulse waveform, designated as “Waveform C” and with correspondingparameters in Table 1, which was also constrained to have an net chargepassed that is equivalent to the DC counterpart described in FIG. 10(“Waveform A”, Table 1). FIG. 12A depicts the discharge segment(negative voltages) during the 4th cycle, which exhibited voltagefluctuations shortly before 500 seconds and continued beyond 1000seconds. The blue colored trace appears to be a solid block, which is amanifestation of blue data points whose interval is significantlysmaller than the plotted x-axis time scale. FIG. 12B exemplifies a smallportion of the voltage transient response, with an x-axis scale thatresolves the individual pulse features centered around 365seconds—significantly before the voltage fluctuations that was mentionedin FIG. 12A. FIG. 12C depicts the onset of voltage fluctuationsexperienced shortly before 500 seconds. The appearance of the voltagefluctuations during cycle 4 suggests that “Waveform C” appeared to haveaccelerated the dendrite-facilitated short-circuit time compared to theconventional DC analog, which experienced the voltage fluctuationsduring cycle 5, as described in FIG. 10. Thus, the choice of pulse/pulsereverse waveforms appear to influence the lithium dendrite behavior.

The following U.S. patents are hereby incorporated herein by thisreference: U.S. Pat. Nos. 9,987,699; 9,006,147; 8,603,315; 7,022,216;6,878,259; 6,863,793; 6,827,833; 6,750,144; 6,558,231; 6,652,727;6,551,485; 6,524,461; 6,402,931; 6,319,384; 6,309,528; 6,303,014;6,210,555; 6,203,684; 6,080,504; 5,084,144.

The following references are also incorporated herein by this reference:Mayers, M. Z.; Kaminski, J. W.; Miller, T. F. J. Phys. Chem. C, 2012,116 (50), pp 26214-26221; Aryanfar, A. et. al. Dynamics of LithiumDendrite Growth and Inhibition: Pulse Charging Experiments and MonteCarlo Calculations J. Phys. Chem. Lett., 2014, 5 (10), pp 1721-1726; Liet al., Sci. Adv. 2017; 3: e1701246; Keil, P.; Jossen, A. Chargingprotocols for lithium-ion batteries and their impact on cycle life—Anexperimental study with different 18650 high-power cells, Journal ofEnergy Storage, 6, 2016, 125-141; Yang, H.; Fey, E. O.; Trimm, B. D.;Dimitrov, N.; Whittingham, M. S. Effects of Pulse Plating on lithiumelectrodeposition, morphology and cycling efficiency, Journal of PowerSources, 272, 2014, 900-908.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention. The words “including”, “comprising”, “having”, and “with” asused herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments. Other embodiments will occur to those skilled inthe art and are within the following claims.

In addition, any amendment presented during the prosecution of thepatent application for this patent is not a disclaimer of any claimelement presented in the application as filed: those skilled in the artcannot reasonably be expected to draft a claim that would literallyencompass all possible equivalents, many equivalents will beunforeseeable at the time of the amendment and are beyond a fairinterpretation of what is to be surrendered (if anything), the rationaleunderlying the amendment may bear no more than a tangential relation tomany equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for anyclaim element amended.

What is claimed is:
 1. A method of electrowinning a metal, the methodcomprising: applying a microprofile charge to the electrowinning cellincluding: forward cathodic pulses to deposit metal on theelectrowinning anode, and between selected successive forward cathodicpulses, applying one or more reverse anodic pulses to polish theelectrowinning anode; and subsequent to the application of themicroprofile charge to the electrowinning cell, applying a macroprofilecharge to the electrowinning cell including: forward cathodic pulses todeposit metal on the electrowinning anode, and between selectedsuccessful forward cathodic pulses applying one or more reverse anodicpulses to polish the electrowinning anode; and said microprofile chargeand macroprofile charge applied to the electrowinning cell sequentially.2. The method of claim 1 in which the microprofile charge includes acathodic to anodic charge ratio of greater than one, an average currentdensity of greater than 5 mA/cm2, a frequency of between 0.10 to 100 Hz,a cathodic duty cycle of less than 10%, and an anodic duty cycle ofgreater than 50%.
 3. The method of claim 1 where in the macroprofilecharge includes a cathodic to anodic charge ratio of greater than 1, anaverage current density of greater than 5 mA/cm2, a frequency of 0.102100 Hz, a cathodic duty cycle of greater than 50%, and anodic duty cycleof less than 10%.